The present disclosure relates to a temperature detecting device configured to detect a temperature of a heated body without contacting, and a fixing device and an image forming apparatus including the temperature detecting device.
In an image forming apparatus, a toner image transferred onto a sheet is heated and pressurized between a heating member and a pressurizing member in a fixing device to be fixed onto the sheet. A temperature of the heating member is controlled based on a temperature detected by a temperature detecting device.
A temperature detecting type of the temperature detecting device typically includes a contact type and a contactless type. As a temperature detecting sensor of the contact type temperature detecting device, for example, a thermistor is used. Since the thermistor is used by being mounted on a surface of the heating member, the surface of the heating member is sometimes damaged by the thermistor. Once the surface of the heating member is damaged, image failure occurs, and thus the damaged heating member must be wasted. In this case, the thermistor as well as the heating member are wasted, and thus problems in cost and resource saving may occur. In addition, there is also a problem that the contact type thermistor is insufficient in precise temperature control because of its slow response speed.
As a temperature detecting sensor of the contactless type temperature detecting device, for example, a thermopile has been known. The thermopile measures a temperature by detecting infrared rays radiated from the heated body, and has a sufficient response speed. In addition, there is an advantage that the thermopile is not required to be positioned in the vicinity of the heating member and can be therefore positioned on a side of an apparatus main body of the image forming apparatus.
However, in such a contactless type temperature detecting device, if an obstacle exists between a temperature detecting surface of the temperature detecting sensor and the heated body, the infrared rays radiated from the heated body are blocked. This has an influence on the detected temperature. In addition, there is also a problem that an output value of the thermopile varies dependent on an atmosphere temperature because of its low heat resistance.
In order to prevent invasion of the obstacle between the temperature detecting surface and the heated body, there is a fixing device including a contactless type temperature detecting device provided with a cover covering the temperature detecting sensor. In the fixing device, by making a positive pressure in the inside of the cover by using an air blowing fan, an adhesion of foreign matters on the temperature detecting surface of the temperature detecting sensor is prevented. In addition, there is a fixing device provided with a pulsatile pump making a pressure in the vicinity of the temperature detecting surface of a temperature sensor higher than an atmosphere pressure. Further, there is a fixing device provided with a guide member configured to guide an air flow formed by an air blowing fan to the temperature detecting surface from the back side of a temperature sensor.
However, in the fixing device provided with the air blowing fan, since the air flow generated by the air blowing fan flows toward a fixing member, the fixing member may be partially cooled, causing non-uniform temperature distribution of the fixing member. In addition, the fixing device provided with the pulsatile pump has problems that the configuration of the fixing device is complicated and cost is increased.